One such can-shaped container of the type described above uses a synthetic resin as the main material. This type has been described, for example, in Japanese Laid-Open Patent Publication No. 39489/1977. Another such cover is described in commonly assigned U.S. Patent Application Ser. No. 614,095, filed May 25, 1984. A similar can is disclosed by Piltz et al, in U.S. Pat. No. 4,210,618.
The present inventors previously proposed a cover as set forth below as a cover constituting such a can-shaped container using a synthetic resin as the main material.
A cover for a can-shaped container is fabricated by preparing, for example, an Al (aluminum) foil having heat-fusible resin layers on both its sides to be flat without being deformed or after being preformed to remain without being substantially stretched. The so prepared Al foil and resin layers are set in advance in a mold of an injection molding machine. After that, a resin is injected to mold a cover by simultaneous injection (integral) molding.
Since the injected molten resin is laminated on the heat-fusible resin layer of the A1 foil in this process, the injected resin layer has high adhesion to the A1 foil and the resulting molded article is free from occurrence of release of the resin layer caused by a heat hysteresis such as occurs in resealing treatment and it also has high strength when dropped. Further, in addition to the above-mentioned advantages, the molded article has advantages that the number of manufacturing steps can be reduced and the manufacturing cost can be reduced by the simultaneous injection (integral) molding.
The same cover as mentioned above can be produced by first molding a resin sheet by injection molding or the like. Then the resin sheet is laminated with an adhesive to an Al foil having heat-fusible resin layers on its both sides. The method of producing the cover by use of adhesive, however, has various disadvantages. Namely, the number of manufacturing steps increases, causing an increase in cost. Food sanitation properties of the adhesive come into question. Also, the resin layer of cover is readily released by the heat hysteresis such as a resealing treatment or the like.
The peripheral flange of the above-mentioned upper cover produced by simultaneous injection molding is fixed to the body part of the can-shaped container which has the same heat-fusible resin layer surface. The fixing utilizes a heat-fusible resin layer disposed on the Al foil on a side opposite to the laminated injected resin layer, for example, by a heat sealing process. In a panel inside a circumferential flange of the cover, there is disposed a cut between the panel and a more interior part. Within the cut, an Al foil having heat-fusible resin layers on its both sides (a multi-layer base) but not being laminated with any injected resin layers is exposed. The cut is configured like a ring with a nearly constant width of the multi-layer being exposed to promote its tearing. The cut is so shaped to make an acute angle at a corner near a point where the opening of can starts. One end part of a grip is fixed to a pedestal comprising an injected resin layer disposed adjacent and inside the cut. Thus, the above-mentioned cover is constructed so that, by lifting the other end of the grip, the exposed multi-layer base material is pierced at a point where the cut makes an acute angle. Subsequently, the multi-layer base is pulled and torn along the cut. As a result, the upper cover produced by simultaneous injection molding is opened.
Further, a lower cover produced by injection simultaneous molding and having a similar construction is fixed to the bottom of the above-mentioned container.
However, the inventors have found that there are the following problems in such can-shaped containers.
Food such as a soup, a cold drink, or the like is filled into the body part of the above-mentioned can-shaped container. The filled containers after being resealed are put into the food distribution chain. In a hot-pack method, contents are filled into a container while they are hot. On the other hand, in winter months, coffee or the like is heated for use at a relatively high temperature in a food sales stand or the like.
As mentioned above, the upper cover and lower cover of the can-shaped containers are produced by laminating an injected resin layer to a multi-layer base having a resin layer disposed on each side of a thin aluminum foil. In the upper cover as mentioned above, there is disposed a notched part (cut) in which the multi-layer base is exposed. Accordingly, cans are likely to leak through pin holes pierced by the acute-angle tip of the pedestal when the can is dropped. Furthermore, at such a high temperature as mentioned above, the multi-layer base exposed by the cut of the upper cover is apt to undergo a deformation or be damaged, in particular, at the tip of the acute angle. In addition to the above, the inventors have found that by the above-mentioned deformation of the multi-layer base in the cut, can strength is lowered when the can is dropped. Further, due to deformation or elongation of the base material in the cut playing a big role when the cover is opened, the cover becomes hard to open or a jagged film remains adhering to an opening, lowering substantially the opening properties of the cover and the product value of the container.